It is well known to represent an image digitally by dividing the image into a large number of segments, denoted pixels, and allocating digital values, denoted pixel values, to each pixel. Typically, the image is divided into a matrix of rows and columns of pixels and the size of a digital image is then given by the number of pixels in a row and the number of pixels in a column. The pixel values are typically stored in an array in a digital memory. For example, a grey tone image may be represented digitally by a digital image comprising pixels each of which has one pixel value representing the grey tone of the corresponding pixel. Similarly, a color image may be represented by a digital image comprising pixels each of which has three pixel values, one for each of the colors red, green, and blue.
Typically, a digital image is created by transmission of light towards an object and detection by an electronic camera of light reflected from or transmitted through the object. However, in general a digital image may be created by transmission of any kind of radiated energy, such as electromagnetic radiation, such as visible light, infrared radiation, ultraviolet radiation, X-rays, radio waves, etc, ultrasound energy, particle energy, such as electrons, neutrons, etc, etc, towards an object for interaction with the object and by detection of energy having interacted with the object, such as by reflection, refraction, absorption, etc.
A digital image may be formed by any imaging system, such as radiometer systems, infrared systems, radar systems, ultrasound systems, X-ray systems, electronic cameras, digital scanners, etc, adapted to detect the kind of energy in question and to generate a digital image based on the energy detection.
The amount of energy needed to record a digital image with a desired signal to noise ratio, i.e. the sensitivity of the imaging system, is determined by the noise level in the imaging system. Energy detectors of any kind generate noise that adds to the signal desired to be recorded. The signal to noise level of a digital image is typically required to be comparable to if not better than the signal to noise level of an image recorded on a photographic film for subsequent reproduction in professional publications.
Typically, an image recording system operating in the visible light range of electromagnetic radiation, such as an electronic camera, a digital camera, an electronic scanner, a digital scanner, etc, uses a solid state imaging device, typically a charge coupled device (CCD), for recording of an image. The CCD is an array of a large number of light sensitive detectors connected to each other as an analog shift register. In each detector of the CCD a charge is formed that is proportional to the light energy incident on the detector during an integration period. The analog charge of each detector is shifted serially out of the CCD and is typically converted to a digital value whereby a digital representation of the recorded image is formed. Each pixel value of the digital image is equal to the digitized charge of the corresponding CCD detector. The pixel values may be transferred to an external computer through a computer interface or may be stored on a memory card or on a rotating magnetic recording medium.
It is known in the art to use linear CCDs in an image recording system in which the linear CCDs light-sensing detectors are arranged in a single line. Typically, the array is moved across the image, scanning it one line at a time. For color images, filters can be placed in front of the array, which then makes three passes across the image during image recording.
Offering a good compromise of image resolution (high pixel count) and cost in an image recording system is the trilinear CCD array, which comprises three linear CCDs positioned side by side. Each line is covered by its own color filter. The array makes a single pass across the image, and each linear CCD records a line at a time.
At a higher cost, a two-dimensional CCD array can capture the entire image in a very short time. Three separate exposures with three color filters can be used to make a color representation. For example, the color filters can be placed in a rotating wheel that sequentially inserts each color filter in the optical path between the image forming optics of the image recording system and the two-dimensional CCD array.
In the present context, the term exposure is to be understood in a broad sense as the time period during which an energy sensor is actually sensing the energy. For example, a photographic film is exposed to light whenever light is incident upon it, while a CCD is exposed to light when the elements of the CCD are allowed to integrate light incident upon them. The CCD is not exposed when its light sensing elements are short-circuited although light may be incident upon them.
It is well known in the art to size a digital image, i.e. to change, typically reduce, the size of the digital image, i.e. the number of pixels of the digital image in order to minimize the amount of pixel data to be stored for later processing of the digital image. Various, sometimes complex, strategies for forming sets of pixels from which new pixel values are calculated may be employed. In a very simple example, a digital image may be down-sized by reducing the number of pixels of the digital image by an integer, e.g. by a number of four. In this case, the pixels of the original digital image is divided into sets of four pixels each and each set of pixels is transformed into a new pixel of a pixel value equal to the average value of the original pixel values. Thereby, a new digital image of one fourth the size of the original digital image is generated.
Further, it is well-known to adjust brightness of an image produced from a digital image by forming a new digital image with the same number of pixels as the original image in which each of the new pixel values is generated by a linear or non-linear transformation of the corresponding original pixel value.
Replacement of conventional cameras for photographing and recording images by use of the photosensitivity of photographic films by electronic cameras has gained increasing attention in recent years.
In photography, it is well known to use photographic films of different light sensitivity and optical resolution. Typically, a photographic film with good light sensitivity has a lower optical resolution than a film with a lower light sensitivity. Simultaneously, the less light that is needed to expose a film the more depth of focus can be obtained. Thus, a photographer is accustomed to work out compromises between the three parameters of a recorded image: resolution, light sensitivity and depth of focus.
In working out such compromises, the photographer uses a light meter. Based on the light sensitivity of the film in the camera and the intensity of light incident on the light meter, the light meter indicates pairs of exposure time and diaphragm settings that will ensure a correct exposure of the photographic film.
It is known to specify the light sensitivity of an electronic camera as a parameter corresponding to the light sensitivity of a photographic film, e.g. as an ASA value. For example, the light sensitivity of an electronic camera may be specified as 400 ASA. This means that a photographer can use a light meter to indicate pairs of exposure time and diaphragm settings for the electronic camera that will ensure a correct exposure of the recorded image when the light meter is set to indicate correct pairs of exposure time and diaphragm settings for a camera with a 400 ASA photographic film.
It is seen that there are resemblances between a digital image, e.g. recorded with a CCD array, and an image recorded on a photographic film. A photographic film comprises light sensitive particles that chemically stores information about light energy incident upon them during an exposure period. By increasing the density of particles on a film, the resolution of the film is increased while the light sensitivity of the film is decreased. Thus, resolution and light sensitivity of a photographic film is interrelated.
Similarly, the number of pixels in a digital image defines the resolution of the digital image and the pixel values of a recorded image correspond to the information chemically stored in the light sensitive particles on the photographic film.
Thus, by analogy with a photographic film, a digital image comprising pixel locations in a memory holding pixel values can be seen to correspond to a developed photographic film while the set of pixel locations in a memory for holding pixel values of a digital image may be said to constitute a virtual film.